So, you've gone from GPS can't do altitude, to GPS can do altitude (albeit, inaccurately).
Your Garmin quote is for consumer GPS - basically, wandering around the hills on an afternoon-type pursuits. I think this is "relative GPS".
Sure, you could locate aircraft with $79 devices as well.
However, I seem to recall that there is a system called "absolute GPS", where (a specialist) receiver is also triangulated against a base station of known exact coordinates. This can give accuracy to millimetres.